Allow setting a flag to control whether or not ComboBox's popup is allowed to obscure the contentItem

Sometimes QQuickPopupPositioner will position ComboBox's popup in such a way that it hides the current item:

import QtQuick 2.12
import QtQuick.Controls 2.12
import QtQuick.Window 2.12

ApplicationWindow {
    visible: true
    width: 640
    height: 190

    ComboBox {
        model: 100
        anchors.horizontalCenter: parent.horizontalCenter
    }
}

It seems some UIs intentionally do this:

However, other styles might not want this, so a flag to control it could be useful.

How it will work is unclear, as the positioner would need to be aware of the contentItem.